There are many types of composite wood products formed from lignocellulosic material. Some examples include medium density fiberboard (“MDF”), oriented strand board (“OSB”), and particleboard. Others include laminated veneer lumber (“LVL”), laminated structural lumber (known in the art as “Glulam”) and plywood. These composite products are generally made by adhering together wood materials such as lumber, veneers, strands, flakes, particles, and fibers with adhesives. Thermosetting adhesives are commonly used, which include phenol-formaldehyde (PF) resins, phenol-resorcinol-formaldehyde (PRF) resins, resorcinol-formaldehyde (RF) resins, urea formaldehyde (UF) resins, melamine urea formaldehyde (MUF) resin and isocyanate (MDI) resin.
Such composite wood products are typically produced by adding the adhesive to the lignocellulosic material and forming a mat of the combination. The mat is heated to a temperature range within which the rate of curing of the adhesive is significant, and is pressed for a period of time, allowing the resin to cure completely, or nearly completely. This process is well known in the art as “hot pressing”. The range of temperature within which the rate of curing of adhesive is significant is referred to throughout this document as the “curing temperature range”. Those skilled in the art of thermosetting adhesives will appreciate that some adhesive curing may occur below the curing temperature range, but that the rate of curing increases dramatically when the adhesive reaches its curing temperature range. The curing temperature range is different for different types of adhesives and may also depend on other factors, such as the type of cellulosic material in the mat, the moisture content of the mat and the vapour pressure within the mat.
For best hot pressing results, it is desirable for the resins within the mat to reach the curing temperature range in order to cure properly. Preferably, the entire volume of the mat will reach the curing temperature range to allow all of the resin in the mat to cure. When heat is applied to the mat in a typical hot pressing operation, however, it is only applied to one or both face surfaces of the mat. In a typical system, the conveyor belt on which the mat rides is heated and supplies heat directly to a surface of the mat. The heat is expected to be conducted from the surface of the mat to the core of the mat. Since wood is not a good conductor of heat, however, this conduction of heat inwardly from the surface of the mat is an inefficient way of heating the core of the mat, and while the resin near the surface of the mat may reach the curing temperature range relatively quickly, it takes much longer for the core of the mat to reach the curing temperature range.
It is desirable to attempt to heat the core of the mat more quickly to save time and production cost in the manufacture of composite wood products. Attempts have been made in the past to add more heat to pressing systems by way of steam injection, and also by way of subjecting the mat to microwave and radio frequency radiation. These techniques are not desirable, however, because they require great amounts of energy, and, especially in the case of radiation, require significant protective mechanisms which are cost-prohibitive.
Accordingly, there is a desire for a system for heating and pressing a mat of lignocellulosic material which permits shorter pressing times.